"Consumers ordinarily expect that natural search results are included and ranked based on relevance to a search query, not based on payment from a third party," the FTC wrote.

"Including or ranking a search result in whole or in part based on payment is a form of advertising. To avoid the potential for deception, consumers should be able to easily distinguish a natural search result from advertising that a search engine delivers."

Although the FTC did not accuse any of the search engine firms of breaking the rules, it warned each of them that promoting its own services in search results constitutes "deceptive practice".

Google has been subjected to several investigations related to its practices, including a two-year FTC probe that cleared the firm of any wrongdoing.